Absorbent articles containing absorbent cores of fluff pulp frequently contain wrinkles in the absorbent core that run from side to side of the article, typically normal to the machine direction in the manufacturing process. Side-to-side wrinkles are deleterious in that they provide large flow channels for urine or other body fluids to escape to the edge of the article. Frequently, such wrinkles are present after manufacturing, before the article has been folded or worn.
Among those working in the art of absorbent article manufacture, several theories have been proffered for the cause of fluff wrinkles. It is sometimes assumed that the wrinkles are the result of unavoidable in-plane compression as tension in the adjoining webs is released. For example, the forming tissue is the layer of creped tissue paper on which the loose fluff pulp pad is deposited in an air-laying process. Tension exists in the forming tissue as it is carried on rolls and between nips, and some tension may be needed for processing. The fluff pad is thus formed on a tissue that is under tension and has been stretched. When the tension is relieved later in the process, the tissue contracts and may cause the fluff pad to contract also, perhaps causing some disruption of the fluff pad. In-plane disruption from stress relaxation may also occur when a topsheet is combined with the fluff pad superposed on a bottom sheet, for the topsheet may be under tension (as well as the bottomsheet or forming tissue) which will be released after it has been attached to fluff. This mechanism has been suggested as the cause of wrinkles in the absorbent core. For example, Weber et al. in U.S. Pat. No. 5,167,897, "Method For Incrementally Stretching A Zero Strain Stretch Laminate Web To Impart Elasticity Thereto," issued Dec. 1, 1992, teach that continuous webs of backsheet material and topsheet material, which are combined with an absorbent core, "are preferably maintained under very slight (essentially `zero strain`) tension in the machine direction to prevent wrinkling and to facilitate registration with the diaper assembly and converting operations until the completed diaper web is severed into discrete diapers." The implication is that strain in webs attached to the absorbent core will cause wrinkling of the core.
Some have assumed that the wrinkles are a result of general and unavoidable in-plane stresses during handling of the fluff pad, particularly during folding and packaging operations.
In examining the physical nature of regular cross-direction wrinkles in commercial diapers, viewing of the fluff pad against strong light from behind allows one to see that most wrinkles occur in zones of low basis weight, as if the pad had been seriously disrupted at that point. Mere folding of the absorbent core in areas of uniform basis weight does not result in low basis weight zones, as is easily verified experimentally. Bending or folding an unwrinkled, uniform region of a pad of fluff pulp constrained between tissues does not result in the types of low basis weight wrinkled areas seen in manufactured articles. Wrinkles are more likely to form during folding in those zones where the pad has been disrupted and has locally low basis weight and thus low stiffness, but the folding is not the cause of the basis weight disruption. Even after the diaper is unfolded and flattened out, apparent wrinkles or regions of low basis weight are evident. These wrinkles are signs of in-plane disruption that occurred during manufacturing. The wrinkles associated with low basis weight zones may compromise pad integrity during use, provide channels for leakage to the side, and disrupt the continuity of the fibrous network and thus interfere with wicking.
While high tension in adjoining webs may impart wrinkling to a fluff pad when the tension is relieved, such wrinkles would primarily be zones of locally increased basis weight as the fibrous web was contracted. In contrast, the wrinkles that are viewed as most harmful to diaper performance are associated with low basis weight regions. Neither relaxation of adjoining web tension nor folding of the diaper during packaging can account for the nature of the most harmful form of wrinkles associated with low basis weight zones. Surprisingly, serious investigations into the cause of wrinkles in the absorbent core appear to have been neglected in prior art, in spite of the long-felt need for absorbent articles that are less subject to leaks. Even though wrinkles are visible in products, previous analysis of leakage and absorbency of diapers and related articles has typically assumed that the absorbent core is a homogenous, wrinkle-free, non-disrupted fluff pad with uniform basis weight and continuous wicking paths for fluid. The invalidity of such assumptions due to the presence of macroscopic wrinkles may partly account for continued high levels of leakage problems in many apparently advanced, highly engineered absorbent articles. Accordingly, an object of the present invention is to provide a means for processing air-laid fluff pulp pads for use in absorbent cores which reduces the number of wrinkles in the core. A further object is to improve core integrity by reduction of disrupted regions of low basis weight running in the cross direction of the core. A further object is to improve longitudinal wicking of an absorbent core by reducing the number of discontinuities in the fibrous medium that could block or disrupt flow of fluids along the length of the core. Yet another object is to provide means for allowing increased production speed of diaper machines and other production devices for absorbent articles such that pad integrity and wicking continuity is not sacrificed at the elevated speed by in-plane disruption of the absorbent core during compressive events.